Commit message: Prevent dereferencing of null pointers in TFLite's `add.cc`.

PiperOrigin-RevId: 387244946
Change-Id: I56094233327fbd8439b92e1dbb1262176e00eeb9
CVE: CVE-2021-37688
CWE: CWE-369
Vulnerability description: TensorFlow is an end-to-end open source platform for machine learning. In affected versions an attacker can craft a TFLite model that would trigger a null pointer dereference, which would result in a crash and denial of service. The [implementation](https://github.com/tensorflow/tensorflow/blob/149562d49faa709ea80df1d99fc41d005b81082a/tensorflow/lite/kernels/internal/optimized/optimized_ops.h#L268-L285) unconditionally dereferences a pointer. We have patched the issue in GitHub commit 15691e456c7dc9bd6be203b09765b063bf4a380c. The fix will be included in TensorFlow 2.6.0. We will also cherrypick this commit on TensorFlow 2.5.1, TensorFlow 2.4.3, and TensorFlow 2.3.4, as these are also affected and still in supported range.

Project: tensorflow
File: optimized_ops.h

inline void BinaryBroadcastFiveFold(const ArithmeticParams& unswitched_params,
                                    const RuntimeShape& unswitched_input1_shape,
                                    const T* unswitched_input1_data,
                                    const RuntimeShape& unswitched_input2_shape,
                                    const T* unswitched_input2_data,
                                    const RuntimeShape& output_shape,
                                    T* output_data, ElementwiseF elementwise_f,
                                    ScalarBroadcastF scalar_broadcast_f) {
  ArithmeticParams switched_params = unswitched_params;
  switched_params.input1_offset = unswitched_params.input2_offset;
  switched_params.input1_multiplier = unswitched_params.input2_multiplier;
  switched_params.input1_shift = unswitched_params.input2_shift;
  switched_params.input2_offset = unswitched_params.input1_offset;
  switched_params.input2_multiplier = unswitched_params.input1_multiplier;
  switched_params.input2_shift = unswitched_params.input1_shift;

  const bool use_unswitched =
      unswitched_params.broadcast_category ==
      tflite::BroadcastableOpCategory::kFirstInputBroadcastsFast;

  const ArithmeticParams& params =
      use_unswitched ? unswitched_params : switched_params;
  const T* input1_data =
      use_unswitched ? unswitched_input1_data : unswitched_input2_data;
  const T* input2_data =
      use_unswitched ? unswitched_input2_data : unswitched_input1_data;

  // Fivefold nested loops. The second input resets its position for each
  // iteration of the second loop. The first input resets its position at the
  // beginning of the fourth loop. The innermost loop is an elementwise add of
  // sections of the arrays.
  T* output_data_ptr = output_data;
  const T* input1_data_ptr = input1_data;
  const T* input2_data_reset = input2_data;
  // In the fivefold pattern, y0, y2 and y4 are not broadcast, and so shared
  // between input shapes. y3 for input 1 is always broadcast, and so the
  // dimension there is 1, whereas optionally y1 might be broadcast for
  // input 2. Put another way, input1.shape.FlatSize = y0 * y1 * y2 * y4,
  // input2.shape.FlatSize = y0 * y2 * y3 * y4.
  int y0 = params.broadcast_shape[0];
  int y1 = params.broadcast_shape[1];
  int y2 = params.broadcast_shape[2];
  int y3 = params.broadcast_shape[3];
  int y4 = params.broadcast_shape[4];
  if (y4 > 1) {
    // General fivefold pattern, with y4 > 1 so there is a non-broadcast inner
    // dimension.
    for (int i0 = 0; i0 < y0; ++i0) {
      const T* input2_data_ptr = nullptr;
      for (int i1 = 0; i1 < y1; ++i1) {
        input2_data_ptr = input2_data_reset;
        for (int i2 = 0; i2 < y2; ++i2) {
          for (int i3 = 0; i3 < y3; ++i3) {
            elementwise_f(y4, params, input1_data_ptr, input2_data_ptr,
                          output_data_ptr);
            input2_data_ptr += y4;
            output_data_ptr += y4;
          }
          // We have broadcast y4 of input1 data y3 times, and now move on.
          input1_data_ptr += y4;
        }
      }
      // We have broadcast y2*y3*y4 of input2 data y1 times, and now move on.
      input2_data_reset = input2_data_ptr;
    }
  } else {
    // Special case of y4 == 1, in which the innermost loop is a single
    // element and can be combined with the next (y3) as an inner broadcast.
    //
    // Note that this handles the case of pure scalar broadcast when
    // y0 == y1 == y2 == 1. With low overhead it handles cases such as scalar
    // broadcast with batch (as y2 > 1).
    //
    // NOTE The process is the same as the above general case except
    // simplified for y4 == 1 and the loop over y3 is contained within the
    // AddScalarBroadcast function.
    for (int i0 = 0; i0 < y0; ++i0) {
      const T* input2_data_ptr = nullptr;
      for (int i1 = 0; i1 < y1; ++i1) {
        input2_data_ptr = input2_data_reset;
        for (int i2 = 0; i2 < y2; ++i2) {
          scalar_broadcast_f(y3, params, *input1_data_ptr, input2_data_ptr,
                             output_data_ptr);
          input2_data_ptr += y3;
          output_data_ptr += y3;
          input1_data_ptr += 1;
        }
      }
      input2_data_reset = input2_data_ptr;
    }
  }
}